1. Field of the Invention
The present invention relates to an air-fuel ratio control device for an internal combustion engine.
2. Description of the Related Art
An exhaust system of an internal combustion engine is usually provided with a three-way catalytic converter which oxidizes CO and HC, and deoxidizes NO.sub.x so that these three harmful materials in the exhaust gas are converted into harmless materials such as CO2, H.sub.2 O, and N.sub.2. The purifying ability of the three-way catalyst depends on an air-fuel ratio of the mixture in an engine cylinder, and it is known that when the air-fuel ratio is stoichiometric, the three-way catalyst can purify all of these three harmful materials at the same time. To counter a variation of the air-fuel ratio, the three-way catalyst usually has an O.sub.2 storage ability such that it absorbs and the stores excess oxygen existing in the exhaust gas when the mixture is on the lean side, and it releases oxygen when the mixture is on the rich side.
By the way, when a temperature of the three-way catalytic converter is very high and the exhaust gas is on rich side, H.sub.2 S gas is generated in the three-way catalytic converter. H.sub.2 S gas has bad smell. When the vehicle running, the gas is emitted in the atmosphere and thus no problem occurs. However, when the vehicle is stopped, the gas flows into the interior of the vehicle so that the driver has an unpleasant sensation. To prevent this problem, Japanese Unexamined Patent Publication No. 62-135626 discloses an air-fuel ratio control device which controls the air-fuel ratio according to a lean target when the vehicle is stopped, i.e., in an idle engine condition.
Once an air conditioner or the like is used in an idle engine condition, the amount of intake air is increased to increase an engine output in accordance with the increment in the accessory load. If, at this time, the above-mentioned air-fuel ratio control device controls the air-fuel ratio according to the lean target, an amount of extra O.sub.2 becomes very large in the three-way catalytic converter, and thus a large amount of O.sub.2 close to the limit of O.sub.2 storage ability is stored therein. Immediately after the vehicle starts again, the air-fuel ratio is controlled to the stoichiometric target to realize a good engine operating condition, i.e., the air-fuel ratio varies with the stoichiometric air-fuel ratio as a control center. Therefore, when the mixture becomes lean, the three-way catalytic converter cannot absorb the extra O.sub.2 so that it cannot deoxidize NO.sub.x sufficiently and thus the amount of NO.sub.x emitted in the atmosphere is increased.